In recent years, thin film solar cells which use p-type light absorption layers constituted by Group I-III-VI2 compound semiconductors or Group I2-(II-IV)-VI4 compound semiconductors have come under the spotlight. Among these, cells which use chalcogenide structure Group I-III-VI2 compound semiconductors which include Cu, In, Ga, Se, and S are called “CIS-based thin film solar cells”. As typical materials, there are Cu(In, Ga)Se2, Cu(In, Ga)(Se, S)2, CuInS2, etc. In particular, cells which include Ga are also called “CIGS-based thin film solar cells”.
Further, cells which use p-type light absorption layers constituted by chalcogenide-based Group I2-(II-IV)-VI4 compound semiconductors which include Cu, Zn, Sn, S, or Se are called “CZTS-based thin film solar cells”. As typical ones, there are Cu2ZnSnS4, Cu2ZnSnSe4, Cu2ZnSn(S,Se)4, etc.
These compound-based thin film solar cells use relative inexpensive, easily available materials, are relatively easy in processes of production, and further have large absorption coefficients in the visible to near infrared wavelength range, so a high photovoltaic conversion efficiency can be expected. Therefore, these are deemed as leading candidates for next generation solar cells.
A compound-based thin film solar cell, in general, is formed by successively stacking on a substrate a first electrode layer which forms a back surface electrode, a p-type light absorption layer, an n-type high resistance buffer layer, and a second electrode layer which forms a light receiving surface side electrode. Here, the n-type high resistance buffer layer can be formed from CdS, ZnS, InS, etc., but from the viewpoint of a good junction property with the p-type light absorption layer, CdS is usually used.
On the other hand, as a defect of a buffer layer which is formed by CdS, it is known that the layer easily cracks. If the n-type high resistance buffer layer has cracks inside it, at the time of forming the second electrode layer, the electrode material will enter into the cracks and a leakage current due to a shunt will be generated between the p-type light absorption layer and the second electrode layer. As a result, the solar cell product will fall in open circuit voltage (Voc) or fill factor (FF). The problem arises that the photovoltaic conversion efficiency (Eff) will fall along with this.
To solve this problem, to raise the function as a buffer layer (suppress shunt between the p-type light absorption layer and the second electrode layer etc.) and improve the photovoltaic conversion efficiency more, a structure comprised of a CdS layer on which an InS layer is stacked is known (see NPLT 1). However, in experiments conducted by the present inventors etc., by making the n-type high resistance buffer layer a stacked structure of a CdS layer and an InS layer, a certain extent of improvement was seen in the photovoltaic conversion efficiency, but the hoped for degree of improvement of performance was not seen. In particular, the problem was seen of a low open circuit voltage (Voc) and a large open circuit voltage loss (Voc, def=Eg−Voc).
NPLT 1. “Influence of heterointerfaces on the performance of Cu(In, Ga)Se2 solar cells with CdS and In(OHx, Sy) buffer layers”, O. Nguyen et al., Thin Solid Films, 431-432 (2003) 330-334